mtd: plat_ram: call mtd_device_register only if partition data exists

mtd_device_parse_register() registers the device as a whole if no
partition data is passed so there is no reason to call
mtd_device_register() after that.

Signed-off-by: Ilya Yanok <yanok@emcraft.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>